U.S. Pat. No. 7,178,353, issued Feb. 20, 2007 and entitled “Thermal Control System and Method”, inventor Kenneth W. Cowans et al and assigned to Advanced Thermal Sciences Corporation, teaches a novel and widely applicable concept for precise and changeable temperature control of a thermal load. Among its departures from other known systems, the system circulates a two-phase refrigerant in direct thermal transfer relation to the load that is being controlled. To do this at different temperatures, it uses a controllable mix of pressurized refrigerant gas at high temperature together with a flow of the same refrigerant, after it has been condensed, then cooled by controlled expansion to provide a flow that is at least partially vapor. The mix then provides a refrigerant flow of predetermined pressure and temperature so that thermal exchange can be effected directly with the load, at a target temperature that can be adjusted up or down. This thermal control is directly effected with refrigerant alone and is therefore more efficient and responsive than most temperature control units, since both pressure and temperature can be controlled with facility, and no intermediate temperature stable media is required.
Consequently, this thermal control technique, which has been descriptively called Transfer Direct of Saturated Fluid (TDSF) is of immediate benefit in a number of demanding applications and also of potentially general capability for a wide variety of temperature control systems. It is of particular promise for applications which require precision control of thermal loads at different temperature levels, along with capability for rapidly varying the temperature levels.
When rapidly shifting between selected temperature levels, however, instabilities and offsets can be encountered since no significant time delays or averaging effects exist in the temperature loop. In systems using TDSF technology, the flow of hot gas controlled by a proportional valve is to be mixed with liquid refrigerant, partially expanded for cooling. While the proportional valve setting can be changed rapidly, imprecision and instability may be encountered because of delays in flow rate variations and system demands. The response times and amplitudes of changes have to be considered in system terms, which factors can be accounted for in accordance with the present invention.
The above referenced patent to Cowans et al, U.S. Pat. No. 7,178,353, also discloses a number of advantageous features within the system, which enhance the ability to separately control pressurized hot gas in one flow path and cold expanding refrigerant in another, before mixing. The patent consequently also discloses a number of techniques for interrupting or modifying flows to increase or decrease temperature particularly rapidly under specified conditions. However, there is often a need for assuring that temperature changes take place at controlled transitional rates that limit overshoot or otherwise provide assurance that a new target has been reached at the thermal load.